Microphone arrays are often used to provide beam-forming for either noise reduction or echo-position, or both, by detecting the sound source direction or location. A typical microphone array has two or more microphones in fixed positions relative to each other with adjacent microphones separated by a known geometry, e.g., a known distance and/or known layout of the microphones. Depending on the orientation of the array, a sound originating from a source remote from the microphone array can arrive at different microphones at different times. Differences in time of arrival at different microphones in the array can be used to derive information about the direction or location of the source. However, there is a practical lower limit to the spacing between adjacent microphones. Specifically, neighboring microphones 1 and 2 must be sufficiently spaced apart that the delay Δt between the arrival of signals s1 and s2 is greater than a minimum time delay that is related to the highest frequency in the dynamic range of the microphone. In generally, the microphones 1 and 2 must be separated by a distance of about half a wavelength of the highest frequency of interest. For digital signal processing, the delay Δt cannot be smaller than the sampling rate of the signal. The sampling rate is, in turn, limited by the highest frequency to which the microphones in the array will respond.
To achieve better sound resolution in a microphone array, one can increase the microphone spacing Δd or use microphones with a greater dynamic range (i.e. increased sampling rate). Unfortunately, increasing the distance between microphones may not be possible for certain devices, e.g., cell phones, personal digital assistants, video cameras, digital cameras and other hand-held devices. Improving the dynamic range typically means using more expensive microphones. Relatively inexpensive electronic condenser microphone (ECM) sensors can respond to frequencies up to about 16 kilohertz (kHz). This corresponds to a minimum Δt of about 6 microseconds. Given this limitation on the microphone response, neighboring microphones typically have to be about 4 centimeters (cm) apart. Thus, a linear array of 4 microphones takes up at least 12 cm. Such an array would take up much too large a space to be practical in many portable hand-held devices.
Thus, there is a need in the art, for microphone array technique that overcomes the above disadvantages.